1. Field of the Invention
This invention relates generally to a chrominance signal processing circuit, and more particularly to an automatic chrominance control (ACC) circuit and an automatic color killer (ACK) circuit for use in a color television receiver, such a circuit being conveniently realizable in an integrated circuit form.
2. Description of the Prior Art
In the chrominance channel of a conventional color television reciever, there are provided an automatic chrominance control (ACC) circuit and an automatic color killer (ACK) circuit, respectively. The ACC and ACK circuits are operated by the color burst signal transmitted with a composite television signal in response to the level thereof, respectively, such that the ACC circuit operates to render the level of a chrominance signal in the composite signal constant, and the ACK circuit operates to render the chrominance channel inactive upon a monochrome television transmission or when the level of the color burst signal becomes lower than a predetermined level and hence to avoid the appearance of so-called color noise on a reproduced picture on a color cathode ray tube.
As an example of the prior art, there is proposed circuitry in which burst level detectors are provided separately as ACC and ACK circuits, respectively. Such prior art circuitry is complicated in its circuit elements and requires the use of separate time constant circuits in producing control voltages for the ACC and ACK operations, respectively, which results in the necessity of a number of external terminals when the circuitry is made as an integrated circuit.
As another example of the prior art, there is provided a common burst level detector for both the ACC and ACK circuits. In this case, the output signal from the common burst level detector is used to control the gain of the chrominance amplifier and to render the chrominance channel inactive upon monochrome television transmission or when the color burst level goes below a predetermined level. This example is of simple circuit construction and requires only one time constant circuit for deriving a control voltage from the burst level detector, so that the circuitry of this example can be easily made as an integrated circuit.
In such prior art circuitry, in order to make the characteristics of the ACC circuit good, the time constant of the time constant circuit must be selected to be relatively long. This will mean that the ACK circuit is not made operative immediately after a color signal transmission is changed to a monochrome signal transmission and, hence, the chrominance amplifier remains operative for some time period after the change of transmission. As a result, color noise appears on the monochrome picture reproduced on the color cathode ray tube for that time period.